The invention pertains to apparatus and methods for cleaning gases, more specifically for removing both particulate contaminants and pollutant gases from the air or other gas to be cleaned, through effects of charged liquid droplets introduced into the gas to be cleaned.
There are numerous industrial processes, and power generation processes used at utility power plants, in which large quantities of air or other gases are used and become highly contaminated with pollutants falling in two broad classes: solid and liquid particulates, and pollutant gases such as toxic or acid gases. For example, both kinds of pollutants will be produced in large quantities by burning of coal at a coal-powered electric power generation facility.
Obviously there is a strong and continuing need to develop improved apparatus and methods for removing both kinds of pollutants from the contaminated air, before reintroduction of the treated air into the atmosphere, with improved pollutant removal efficiency; and to achieve that end by means which are more practical, reliable, and reasonable in cost.
It is known in the gas cleaning arts to remove particulates from the gas to be cleaned by an approach in which the particulates are first electrostatically charged by passing the gas containing the particulates through a suitable electrostatic charging apparatus, and then introducing into the gas to be cleaned numerous charged liquid droplets having electrical charges of polarity opposite to that of the charged particulates. In this approach the gas cleaning process proceeds, as to the particulates, through the effect of electrostatic attraction between the oppositely charged droplets and particulates, a monopole--monopole attractive force, inversely proportional to the square of the droplet--particulate separation distance and proportional to the product of the charge magnitudes, which force draws the particulates in for absorption by the liquid droplets. The droplets are then removed from the air stream, with the collected particulates, by processes well known in the art.
It is also known in the art to remove pollutant gases from an air stream through introduction of liquid droplets, by the process of adsorption of the toxic or acid gases by the liquid droplets, as their molecules come into contact with the surfaces of the droplets, followed by subsequent conventional removal of the droplets and collected pollutant gases from the air stream.
It is desirable to provide an apparatus and method allowing removal of uncharged particulates from the stream of gas to be cleaned, since this would obviate any need for preliminary electrostatic charging of the particulates. The present invention accomplishes this through introduction, into the gas to be cleaned, of liquid droplets which are sufficiently highly charged, and have sufficient number density, to achieve high efficiency particulate collection of even uncharged particulates, by means of a monopole--dipole interaction, instead of the monopole--monopole interaction between charged droplets and charged particulates, used in the conventional approach.
In the present invention, each highly charged liquid droplet induces an electric dipole moment in each nearby uncharged particulate. As is well known, there is an attractive force between an electric monopole and a nearby induced electric dipole. So in the present invention, the particulates are drawn to the charged droplets by the monopole--dipole force, rather than the monopole--monopole force which acts in apparatus using the conventional approach. Since the particulate is uncharged, collection of the particulate by the liquid droplet causes no change in the magnitude of the droplet charge, so that there is no reduction in the magnitude of the dipole moment inducing capacity of the droplet.
As detailed below, the magnitude of the monopole--dipole force has a markedly different and far more sensitive dependence on the droplet--particulate separation distance, and also a more sensitive dependence on the droplet charge magnitude, than the monopole--monopole force active in the conventional apparatus.
So, the present invention must meet the related needs of providing, in the gas to be cleaned, an adequate number density of adequately charged liquid droplets, in order to produce high efficiency of particulate cleaning through use of the monopole--dipole force.
And, there is a need for such an invention which also can achieve a sufficient efficiency of removal of pollutant gases from the gas to be cleaned, by providing, within the gas to be cleaned, a suitable number density of the liquid droplets and a sufficient surface area of the droplets, to provide sufficient liquid droplet surface area per unit volume, to allow the required adsorption of the pollutant gases by the liquid droplets. As detailed below, the present invention also meets this need, with the same droplets which are adequate for achieving high efficiency of particulate collection through the action of the monopole--dipole forces between droplets and particulates.
It is desirable that such an invention be able to maintain constancy of the liquid droplet charge magnitude, as the droplets interact with the gas to be cleaned, so as to maintain the particulate collection capacity of each droplet during such interaction, rather than having that capacity be depleted as would occur if the droplet charge were to be depleted.
And there is a related need for such an invention which can achieve the needed degree of charging of the liquid droplets without the use of very high voltages which are often required in electrostatic precipitator machines, e.g. voltages of the order of 60 kv, since such voltages are apt to cause corona discharges in the gas to be cleaned, which may well deplete the droplet charges, as well as producing other undesirable effects, including space charge effects which may interfere with adequate charging of the liquid droplets.
The present invention apparatus fulfills this need through the use of electrode geometry at the site of droplet production and charging, involving spreading liquid sheet electrodes emitting the droplets from the edges of the liquid sheets, interspersed with metal induction electrodes, with electrostatic potential of no more than about 20 kv existing between the induction electrode array and the array of liquid sheets, and with spacing such that adequately high electric field strength can be maintained at the edges of the liquid sheets to allow adequate charging of the droplets emitted from the liquid sheets, without the occurrence of corona discharges.
As further detailed below, other needs met by the present invention include, without limitation: allowing the particulate and pollutant gas scrubbing procedures to be carried out simultaneously in a single chamber; requiring no significant power other than that for the blower or other means which moves the gas to be cleaned through the cleaning chamber; and allowing these results to be achieved with low liquid-to-gas flow ratios.